Newton wrote a book, Opticks, on the subject-it's useful to look there.

The photon doesn't ``know'' nor does it ``need to know'' anything. That's just sloppy language. The resolution was provided by quantum mechanics, already, but  Feynman developed the spacetime picture:  the contribution to any amplitude involves a sum over all paths, weighted appropriately.  in the classical limit only the classical trajectory contributes. 

You will need a very long wave pulse with very coherent light to achieve such long-distance interference. Feynman's estimate was presumably based on the achievable properties of lasers at his time; that could be even larger now.

Dear Ilian,

How does the photon know? is a very embarrassing question, yes  it's a sloppy language but has truth in it: suppose the first reflecting surface is at 1 m from the source if we start at t=0, at approximately t=1/c the photon reaches the first surface then at some time between 1/c and 2/c we add the second reflecting surface. Do we  get the same pattern as if both surfaces were there the whole time?.

Interference experiments take place over long times. Relativity would prohibit the transmission of information faster than light: if, for example, the position of one of your reflecting surfaces were suddenly changed, then this could not affect what happens at the other surface until a certain time later. But interference, which is stationary, does not in any way conflict with relativity.

Thank you Stam,

about Newton Opticks. I found it but its very difficult to locate the experiment in question. I must read (I hope) chapter 1 but it will take a full free day or so. My question was rather - were his results confirmed by others, because I can not find anything not even in todays literature. It can be just a legend what he says and Feymann cites.

About the wording photon doesn't ``know'' nor does it ``need to know'' "" that's not a point at all for me. I see it does not sound very scientifically but I dont want to write extensively in order to adhere to non - sloppy language,

I dont think QM provides a resolution whatsoever, it just calculates the results (of course very well).  If the Newton experiment is true - I can not see how the experiment and QM is not contradicting  SR.

Thank you Kåre,

I always struggled with this concept of coherent light. If one considers waves its OK, if it is only classical physics its OK . But the light is made of particles with the probality wave attached to it (QM) and everything should be explained from this point of view!!! Not classical physics viewpoint for EM waves!!!

So what does "a very long wave pulse with very coherent light" mean for a particle?

Secondly you write "that could be even larger now" = OK - give me the source!!!

Thank youi Faical,

You wrote "suppose the first reflecting surface is at 1 m from the source if we start at t=0, at approximately t=1/c the photon reaches the first surface then at some time between 1/c and 2/c we add the second reflecting surface. Do we  get the same pattern as if both surfaces were there the whole time?"

I think that only the moment when the photon reflects is important. The second surface must be there or not at this time (taking Heisenberg into acount anyway). But that's in fact a matter of experiment = regretfully I can not find anything experimentally and theoretically about this.

Thank you Leyvraz

You wrote:

1. "Interference experiments take place over long times."

Are you sure of this? If we use many photons simultaneously the time would not be more than x/c.

2. "Relativity would prohibit the transmission of information faster than light"

Surely it must!!!  - but this case does violate it! The info is on the second surface (1 - there is a surface, 0 - there is not). At the moment of the hitting, the point where the photon will go is specified (trying not to use "the photon knows where to land on a screen").

3. " if, for example, the position of one of your reflecting surfaces were suddenly changed, then this could not affect what happens at the other surface until a certain time later."

I don't object this!

4. But interference, which is stationary, does not in any way conflict with relativity.

I wouldnt like to comment on this.  

I could imagine there is a conflict between the concept of "photon" (as an eigenmode of the electromagnetic field with Eph=\hbar\omega) and the notion of the photon being somewhere (such as "at surface 1" or "at surface 2") at some specific point in time. A "localized photon" cannot be monochromatic and imho there is a clash of classical pictures immediately. I would tend to think that this might just be another way of making F. Leyvraz' statement on long time scales involved with interference.

@Leyvraz

What about one single photon interfering with itself?

`` At the moment of the hitting, the point where the photon will go is specified (trying not to use "the photon knows where to land on a screen")''

I assume you mean ``at the moment of hitting the first screen''. If that is so, then I do not think this to be correct. Worse, I do not think there *is* a moment of hitting the first screen: a photon is delocalised, and in order to produce such an effect, it must be considerably so (see Kai Fauth). So we cannot say when the photon hits the screen. Quantum objects are somehow intermediate between waves and particles (more exactly, they are like neither) and your analysis is too much in terms of a particle being at a specific place at a specific time. Photons just are not like that.

OK Levraz, you are referring again to sloppy language "hitting". What will change if I say interferes. (By the way how do you imagine a non-localized object which is registered in a point?).

My purpose is not to clear the model of EM radiation, light or photon. 

What I want to know is: 

are there other experiments like the one described by Newton and similar with lasers as Feynman mentions or this is just a legend.

Or in other words is there interference with thick glass observed?!!!

I think it is possible to make the same kind of experiment with two thin glasses (instead one very thick - this would be much easy!). If we move the second surface with step lambda the reflection from the first must change from 4 to 16 %.

I would appreciate an opinion on such a setup!!!

At the first place, a photon is not a wave. It is a string of equidistant shock fronts that I call warps. Warps are solutions of a homogeneous partial differential equation. Such solutions are known since two centuries. You can look them up. (https://en.wikipedia.org/wiki/Wave_equation#Algebraic_approach See u(x,t) or take a look at https://en.wikiversity.org/wiki/Hilbert_Book_Model_Project/Quaternionic_Field_Equations#Solutions)

The path of the photon passes through a field that guides the photon via its deformations. This field knows the spatial configuration even when it is constituted by fifty meters of glass. The warps are vibrations of this field and passively follow the deformation. 

By the way, the field is not the EM field! It is the field that massive particles deform.

https://en.wikiversity.org/wiki/Hilbert_Book_Model_Project

Ilian> Or in other words is there interference with thick glass observed?!!!

I don't think anybody have tried with 50 meter thick window glass, but much more likely with glass in the form of fiber optic cables. There is a lot of experimental work done in optics, including single-photon interference. Of which I am mostly ignorant. But some Google searches could be a good way to get started.

I see what you mean. I tried to google "experiment reflection fiber optics and surface Feynman" and other similar options. The results are about total internal reflection mostly. 

I think the interference pattern observed by Newton is mainly caused by different photons marching in synchrony; hence the interference is more of the type one can observe at military parades by well-trained troops. A participant turning around at a certain point learn of what is going on 50 meters further down by observing those returning from there. Of course, a classical wave does not have to observe anything; it only has to be coherent to form an observable interference pattern.

Physics becomes more weird/interesting when the light intensity becomes so low that each photon can only interfere with itself; then it must move two (or more) ways at once. I suspect it is more difficult to observe interference over long distances in this case. How "long" can one make a single photon? For a laser, how does that compare with its coherence length? I don't know (since such information was more than one web-search away).

To observe interference I believe the most common method is to send the light through a Mach-Zender interferometer. The main purpose is to make the photons travel different distances, not disturbing or loosing to many of them along the way.

http://www.toptica.com/fileadmin/Editors_English/12_literature/quantum_technologies/12_orders_of_coherence_control.pdf

``What will change if I say interferes. (By the way how do you imagine a non-localized object which is registered in a point?).''

Try to change the way in which you say it, and see if it is still paradoxical. In my opinion, what caused the problem concerning relativity in your mind, was precisely the idea that the photon *is* somewhere, and has to learn about what happens elsewhere. This is not the way to conceive interference. Rather, you should think of the wave as being everywhere, and different parts of it contribute to the value observed.

Maybe thinking in purely classical terms can help: surely, a light wave interfering with itself cannot contradict relativity, since it is essentially described by it. Now a light wave is simply an extended field pattern. We tend, using geometrical optics, to say that part of this pattern went along one path, another part along another. But all this is approximate, and the field pattern forms an indivisible whole.

However, particularly when a ``light ray'' appears to be split, we speak of interference, meaning that the result, when the ``two rays recombine'' can generate oscillations and interference fringes. It is a good way to understand in approximate terms what happens: phase coherence was kept along each ray, and as the rays recombine, this influences the result. But it is still fundamentally approximate.

There was no issue of one ray knowing where the other was: in fact, rays are constructs of the human mind. And in quantum mechanics, the photon is also a coherent, indivisible entity, which it is not useful to separate into rays.

As to experimental realisation, I would say that you are thinking of what is known as a Fabry-Pérot interferometer, which is commonly used for high precision experiments. This can certainly be searched for on the web.

What will be your last opinion - is the experiment of Newton with 35 000 wavelengths thick glass real or not????

Since a) 35 000 wavelengths of visible light is about 2 mm, a reasonable thickness of glass at the time of Newton, and b) the story was told by Feynman, my opinion is that it was real.

Why don't you try deep clear water? It is easier obtainable.

It is 17 mm by 500 nm. Do you think I can see 4 to 16 % by moving two thin glasses?

"Why don't you try deep clear water? It is easier obtainable." - GLASS IS ALSO easier obtainable AND MORE STABLE 

When trying very long distances in glass then the (in-) homogeneity of the refractive index might start to play a role on some scale. For the last generation of refractive optics wafer steppers (usng larrge diameter and thus fairly thick lenses, several cm) e.g., a homogeneity of 10-8 had to be achieved for reliable wafer illumination (sufficiently well defined wave fronts).

A well-known experiment to change the effective path length through glass is to take a not so thick plate and to rotate it. We used to have that in one of our lab courses. And yes, you get nice intensity oscillations as a function of the angle at which the laser beam hits the glass.

Whether the story of Newton is real or not is somewhat besides the point: Fabry-Pérot interferometers work on the principle of interference inside a dielectric block. You do not need such long distances, but you can get sharp interference corresponding to several reflections.

Leyvraz, so you think it is easy to receive intereference with two transparent glasses set say 10 cm apart, because it is a Fabry Perot like?

I am not quite convinced that in FP only the closest (next) paths interfere and then sum up after the lense. When we have 4 % reflectiion then an intreference just one time makes 16 %. If we have multiple reflexions this can exceed 100%? Quite strange - I can think a way out right now.